Hubble Space Telescope
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Hubble Space Telescope
Hubble Space Telescope(HST) The first of NASA's Great Observatories, named after the US astronomer and cosmologist Edwin Hubble, and also the first mission in that agency's Origins Program. It was launched from the space shuttle Discovery in Apr. 1990 into a 600-km low-Earth orbit and in a projected 15-year mission was set to provide extensive imaging and spectroscopic observations over ultraviolet, optical, and near-infrared wavelengths. ESA's participation gave European observers at least 15% of the available observing time.
The science instruments carried into space aboard the HST included a 2.4-meter-diameter telescope and, in its focal plane, five highly sensitive scientific instruments functioning in the range 115–1000 nm. These were (1) the wide field/planetary camera (WF/PC), operating over 120–1000 nm; (2) ESA's faint object camera (FOC), operating over 120–650 nm; (3) a high-resolution spectrograph (115–320 nm); (4) a faint object spectrograph (115–700 nm); and (5) a high-speed photometer. In addition, one of the fine-guidance sensors made accurate astrometric measurements. The pointing accuracy was 0.007 arcsecond. Power was provided by solar arrays, and communication was made to Earth using relay satellites.
Free from the distortion and absorption effects of the Earth's atmosphere, the HST was expected to send back images that would have a resolution of 0.1 arcsecond or better. But initial observations revealed a flaw – spherical aberration – in the primary mirror. This turned out to be the result of a manufacturing error. Most of the scientific programs originally envisaged, however, were carried out. There was important new spectroscopy and imaging of hot stars, late-type stellar chromospheres and coronae, the interstellar medium, galaxies and clusters, quasars and active galactic nuclei, and solar-system objects.
A servicing mission to the HST – the first of four – took place in 1993 using the space shuttle Endeavour, during which a corrective optics package, known as COSTAR, was emplaced and other repairs made. Pairs of mirrors in the COSTAR module focused light to five apertures serving three of the instruments – the two spectrographs and the FOC. COSTAR itself took the place of the high-speed photometer. The WF/PC was also replaced by the improved WF/PC-2. Sharply focused imaging began in 1994 and soon produced dazzling results. During the second servicing mission, by Discovery in 1997, a Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and Space Telescope Imaging Spectrograph (STIS) replaced the previous high-resolution and faint-object spectrographs.
In October 1997 NASA announced that the HST mission would be extended another five years, taking its projected operational life up to 2010. In 1999, however, the cooling system for the NICMOS ceased to function and the instrument stopped working. More importantly, one of the HST's gyroscopes failed and the telescope could no longer be aimed. Astronauts crewing the space shuttle Discovery repaired the gyroscope, updated the onboard computers and carried out maintenance work. The telescope was upgraded again during a fourth service mission by the space shuttle Columbia in March 2002, with the installation of new solar arrays and a power control unit, a new cooling system for the NICMOS and a new scientific instrument, the Advanced Camera for Surveys (ACS). A fifth mission was cancelled because of safety concerns arising from the loss of Columbia in 2003. Any further missions were postponed indefinitely after NASA decided not to prolong HST's operations beyond 2010.
Despite its faults and problems, the HST proved one of NASA's outstanding successes. From 1994 it produced a long succession of sharply focused, beautiful photographs, showing in unprecedented detail a range of objects from nearby planets and asteroids to distant galaxies and gas clouds. Its famous Hubble Deep Field image of 1995, a 10-day exposure of a tiny patch of sky, revealed 1500 galaxies in various states of evolution and gave astronomers their best view back in time to the early dawn of the Universe. The HST discovered black holes at the centers of galaxies and made reliable distance measurements to galaxies using Cepheid variables as its yardsticks. In these ways, the HST helped researchers to peer into the Universe's deepest mysteries.
See also James Webb Space Telescope.